ABSTRACT The factors essential to induction of a protective immune response to bacterial infections are incompletely understood. In the case of Listeria monocytogenes (LM) infection, a category B Priority Pathogen, CD8 T cells are important in effecting sterilizing immunity. LM interfaces at multiple levels with vaccinology as well as the Biodefense initiative due to the significant potential of LM as a vaccine candidate but also as an agent for intentional contamination and generation of highly virulent recombinant strains. Our work has begun to examine the parameters required for effective immunization with killed or mutant LM as potential vaccine vehicles. Specifically, immunization with heat killed LM (HKL) or irradiated LM (IRL) induced distinct patterns of T cell activation characterized by much more robust CD8 T cell memory induction following vaccination with IRL. However, IRL immunization did not induce the level of T cell activation and memory induction obtained with live LM infection. Thus, this is an excellent system for delineation of the mechanisms involved in promotion of distinct degrees of CD8 T cell activation generated by distinct forms of the same pathogen. Based on our preliminary data, the newly proposed studies focus on integrating the anatomy of effective CD8 T cell immune response initiation and memory T cell reactivation with the costimulatory and genetic programming necessary to drive a protective immune response. The hypothesis that responses induced by live infection versus immunization with attenuated vaccines share common essential components to generate protective memory will be tested in three aims: Specific Aim 1. To determine the role of costimulation and CD4 T cell help in priming and protective recall responses following LM infection or vaccination. Specific Aim 2. To determine the anatomical features of effective CD8 T cell vaccination and recall responses. Specific Aim 3. To define the genetic signature of responding CD8 T cells in response to effective vaccination.